This invention relates to an improved process for making diaryl alkylphosphonates and oligomeric and/or polymeric arylene alkylphosphonate derivatives thereof. The diaryl alkylphosphonates and their oligomeric and polymeric derivatives are useful as flame retardants, e.g., in epoxy resins intended for electrical and electronic applications.
Flame retardancy of electrical and electronic circuitboards (printed wiring boards, PWB) is desirable, and for many such products, firmly required. Underwriters Laboratory (UL) flammability test methods are commonly used, a frequently encountered requirement being passage of the UL 94 test with a V-0 rating which signifies a specified fast extinguishment after ignition by a standard flame. It must be noted that thermomechanical and electrical properties must not be unduly compromised by the flame retardant. Electrical resistance, dielectric properties and the integrity of the copper-to-resin lamination must be retained. Thermal stability during the soldering operations must be adequate.
Many of the printed wiring boards used at present are based on epoxy resins, often glass-reinforced, with laminated copper conductors. It is common practice to include a bromine-based flame retardant, most typically tetrabromobisphenol A, which is reacted into the epoxy resin. Due to environmental concerns over the disposal of scrap electrical and electronic products as well as other similar concerns, there is a strong industrial interest in avoiding the use of bromine-based flame retardants and finding alternatives thereto such as phosphorus-based flame retardant materials.
The use of aromatic alkyl phosphonates as flame retardants in epoxy resins is known. However, the complexity of the multistep processes used for making these phosphonates have been a drawback. Also, the presence of catalyst residues in the product phosphonates has tended to compromise the electrical properties of the flame retarded PWB. It is known that electrical resistivity and resistance to dielectric breakdown are reduced by ionic impurities (“Plastics for Electronics,” M. Goosey, ed., Kluwer Academic Publishers, Dordrecht, Netherlands, 1999, pp. 304-5). Other disadvantages of having high ionic (as exemplified by sodium) catalyst levels in an epoxy resin formulation are cited in U.S. Pat. No. 6,037,425, namely, excessively fast cure and difficult temperature control.
The preparation of diphenyl methylphosphonate by reacting triphenyl phosphite with methanol in the presence of a catalytic amount of methyl iodide is known from E. M. Honig and E. D. Weil, J. Org. Chem. 42, 379 (1977) and from U.S. Pat. No. 4,377,537. In the process described by Honig and Weil, phenol by-product is formed and must be removed. A more troublesome by-product, the very unstable diphenyl hydrogenphosphonate, is also formed and an extensive multi-step washing procedure is required for its removal. The process typically provides a rather poor yield of product and generates an excessive amount of waste.
U.S. Pat. No. 4,377,537, the entire contents of which are incorporated by reference herein, describes the reaction of methane phosphonic acid dimethyl ester (i.e., dimethyl methylphosphonate) with triphenyl phosphite in the presence of a relatively large amount of methyl iodide (a volatile, costly and highly toxic compound) as catalyst to provide methane phosphonic acid diphenyl ester (i.e., diphenyl methylphosphonate).
U.S. Pat. No. 4,374,971, the entire contents of which are incorporated by reference herein, describes the transesterification of diaryl alkylphosphonate with aromatic diol and, optionally, a branching monomer, in the presence of a neutral ester interchange catalyst such as a C1-C18 tetraalkyl titanate, dialkyl tin oxide, dialkyl-dialkoxy tin compound, C3-C18 tetralkyl zirconate, C2-C18 trialkyl vanadylate, antimony salt, bismuth salt, C2-C4 dialkyl stannic acid ester, C2-C4 trialkyl stannic acid ester or mixture of germanium dioxide or titanium dioxide with at least one of the foregoing to provide aromatic, optionally branched-chain, polyphosphonates (arylene alkylphosphonate polymers) having a number average molecular weight (Mn) of 11,000 to 200,000.
U.S. Pat. No. 4,690,964, the entire contents of which are incorporated by reference herein, describes the transesterification of diaryl alkylphosphonate with aromatic diol and, optionally, a branching monomer, in the presence of an alkaline catalyst such as an alkali metal and/or alkaline earth metal alcoholate, phenolate, oxide, amide or salt to provide branched or nonbranched oligomeric polyalkylphosphonates (arylene alkylphosphonate oligomers) having a weight average molecular weight (Mw) of about 2,000 to about 10,000.
International Publication No. WO 03/029258, the entire contents of which are incorporated by reference herein, describes the transesterification of diphenyl alkylphosphonate with aromatic diol in the presence of a catalyst such as sodium methylate to provide hydroxy-terminated oligomeric phosphonates useful as reactive fire retardants for epoxy resins.
The oligomer/polymer products provided by the processes of U.S. Pat. Nos. 4,374,971, 4,690,964 and WO 03/029258 may contain amounts of metal catalyst residues which, if not removed (a difficult procedure which adds to cost and because of the high viscosity of the oligomer/polymer, requires special equipment), can be deleterious to the demanding residual electrical properties required for printed wiring board applications.
It is an object of the present invention to provide a high-yield process for the manufacture of diaryl alkylphosphonate.
It is a further object of the invention to provide such a process with little or no requirement for purification steps, and in particular, little if any need for washing and distilling.
It is yet another object of the invention to provide a process for making diaryl alkylphosphonate directly usable for further reaction with aromatic diol to provide an oligomeric and/or polymeric flame retardant exhibiting outstanding properties for use in electronic and electrical printed wiring boards.